Secondary Battery Installation

Background:
My car is supposed to shut the headlights off when the car turns off. On occasion, it doesnt and my battery can't start my car. A jumper pack would alleviate this and I could take care of the situation myself. However, they are $60-150 and id rather invest that money into something multifunctional.

Thoughts:
Install a secondary battery in the trunk with an isolator/relay installed between the two batteries. The battery in my trunk would be used to keep my computer in sleep while the car is off (Intel Atom, SSD, 2 memory sticks). If my car decides to not shut off my headlights again, I would then be able to disconnect everything from the secondary battery and use jumper cables to jump my main battery. I would do it with jumper cables because the main lines I have between the batteries are not large enough to send the cranking amps without the potential of melting something/starting a fire.

Questions:
Am I missing anything?
What type of battery should I use? (SLA/AGM/Deep Cycle/Starter/# Of AH)
How do I calculate the amperage drawn while my computer is sleeping? (Or would getting something to measure it be easier, needed for AH)

Personally I would fix the problem with the lights or the switch or the sensor. If it drains the battery, it will only take 2-5 times of complete draining to kill that battery for good. For your computer I would get a deep cycle for the trunk, keep in mind that a deep cycle is not a starting battery. It would get you out of trouble, but too many jump starts and you will be replacing the deep cycle eventually. Without fixing the original problem you will get stuck in a vicious cycle of buying batteries. Just my 2 cents SNO

And I agree about NOT getting a jumper pack. They usually only have a 12V AGM of about 15-18AH (early packs might use 7AH) and it's much cheaper getting just the battery - whether an isolated second battery or a standalone.

And good to see you understand the limitation about manually connecting the isolator for a jump start (which only requires a switch and 2 diodes for the UIBI or charge-light controlled relay/isolator, and also for many voltage sensing isolators - ie, where you can intercept its control wire to the relay/solenoid). However... If you use a 30A isolating relay & 30A wire with 30A self resetting breakers at each end, then IMO there is no harm using it to try to "jump" start - unless the main battery is really flat. (This is more for flatteries that need just a bit more juice.)
If it doesn't work, you can manually jump start. The breakers should reset within minutes if that.
If you are using a larger relay and cable, it's even better. My set up usually uses a 60A relay, some very high rated cable, and 50A self resetting breakers and I used to manually switch on the UIBI/relay when my aged cranker had problems. But my (reduction) starter motor is only 140A compared to the OEM starter of ~240A.
Some might argue jumping puts stain on the relay, but a 30A is cheap. My 60A relay is only ~$10.

And unless your interlink fuses are of quite high rating, I'd strongly suggest self resetting breakers. Despite at most an 8A load, my 30A fuses used to blow as a result of the 2nd battery's initial inrush current if it was slightly discharged.

Of course that's only if you fit a 2nd battery after fixing the headlight problem. But a 2nd battery for PCs and independence of the main cranking battery is real nice. Plus it can be a good emergency battery (eg, 15AH or larger, though 7AH may suffice depending on the vehicle etc).

Sno's suggestion would have been my suggestion too... Fix the problem or it will cause you other grief down the road.

Every time you fully discharge a lead acid battery you half its life. So Sno is accurate with his number and that is for a brand new battery.

You could get a device something like this:http://www.prioritystart.com/
I have seen something that would work well for you called "Battery Buddy" but I can only find pictures of it on the net. It is a similar device but the idea is that if your battery drops to a dangerously low voltage the unit will disconnect the battery saving you from having to replace it as well as allowing you to start it. This or any other option should be used as an "insurance" policy to save your vehicles battery and to give you the ability to always start. The battery buddy requires you to hit a reset button to start your car. The price should be in the neighborhood of $50-$80 but it is piece of mind if you have issues like this.

You CAN add a second battery but if you don't need it don't add it. Unless you have a carPC or stereo system you play a lot with the vehicle off you really don't need a second battery. Since CarPC automotive supplies have shut downs based on timers or battery voltage you should make use of them instead of a second battery. You could always as a back up put a small UPS style battery in front of your CarPC. You can get chargers for these batteries that hard wire into your car and then you run the PC off the battery. Just make sure you run a wire from the ground of this second battery to ground to make sure your computer works when connected to the rest of the vehicle if you are hooking to any components powered by the normal vehicle power. I would make sure the battery is sized big enough to be able to run your PC until the timer times out which is totally dependent on how you have it setup.

Beyond that the battery buddy or similar device might be helpful in your situation but realistically you need to get your issue repaired regardless. Also as far as a carputer is concerned most automotive PC supplies should have a shut off on them to kill your computer before it kills the battery. Most do it by timers.

If you DO decide to add a second battery you should go to a respectable RV dealer. They will have lots of experience with multiple battery setups and know what works with what. They can provide you with pro's and con's to what is currently available. There are difference of opinions between people on this system of what the best setup is so I strongly suggest an RV dealer for the current scoop regardless of what is said here. I would not trust a car audio dealer necessarily because their experience will be much more limited and they may tend to steer you towards more expensive products or ones that simply don't work. RV dealers are a totally different animal and in the long term you are more likely to be happy and spend less money on something that works.

For the simplest dual battery setup you want a constant duty solenoid. Standard relays tend to not be as reliable and can't handle the momentary surges you are likely to come in contact with. A constant duty solenoid is a special purpose solenoid built similar to a starter solenoid but where as a starter solenoid is designed for high amperage, low duty (short time on), a constant duty solenoid is designed to run fully with the car. Plus if you want to jump the car you can normally just place a very short small jumper from the good battery terminal to the "on" terminal on the relay and you are "jumped". I will end this by stating in a 2 battery system you need batteries that are matched. All this means is you want batteries designed to run together. The closer the match the better life you can expect from the batteries. if they are not matched you could end up with a reduced life on one of the batteries or it may never be fully charged. There are other types of battery isolators but they each have their pros and cons.

That prioritystart is a discontinued product (I wonder why?). It also has an 8-12mA drain.

Two batteries connected with a relay are seen as one battery by the alternator.
Even if there were path resistances, that has no appreciable effect unless the voltage drop is permanent and below battery requirements. And that only happens for improperly sized interlink cabling (which is common is Australia so that companies can sell $200 dc-dc converters to the ignorant).

Basic theory - and hopefully common sense - dictates that 2 loads connected in parallel see the same voltage.
If a battery drops below 13.8V due to an initially high recharge current thru its cabling resistance, that current gradually drops and then the normal load (and battery recharge else float current) means a return to its proper voltage. (Again, assuming proper design, and of course a typical modern alternator with 14.2-14.4V output.)

And yes, I am the said "other member". The above theory comes from basic texts (Ohm's Law and basic circuit/network theory (Kirchhoff, Norton, etc)) and a host of sites like the12volt, bcae1, etc, as well as (in my case) secondary education. The battery stuff comes from professional experience and is supported text and sites like batteryFAQ, IEEE, theBatteryUniversity, etc.
But feel free to discuss it with redheadedrod's school teachers. Just make sure you specify that this is when charging the batteries involved.

Not going to fully bite OS... But the differences you and I have are based more on opinion than fact since I have also used many of the same sources for my information and research done talking to engineers in companies that make those products. And I will leave it at that.

And one note about batteries in parallel. A voltage sensing alternator is going to modulate its output voltage based on the voltage of the battery it sees. With batteries in parallel the voltage seen will be of the highest voltage battery. With this in mind if your alternator shuts down the output voltage because the most charged battery is charged then the lesser charged battery will never get a full charge if the alternator drops below the "charge" voltage. Thus the second battery never gets fully charged. This is why batteries need to be matched even in parallel. If you are able to otherwise sense the batteries voltage and modulate the output to each battery depending on load required then you can properly charge both batteries without over or under charging either. However as pointed out in the past devices that do this are hard to come by and when you do find one they are generally expensive. In general usage however choosing similar batteries should be good enough. As an example Just be aware that if you commonly discharge a deep cycle battery and put it in line with a normal battery that stays mostly charged that this deep cycle battery may never fully be charged.

All alternators on cars built today and probably last 20 years are voltage sensing but not older cars. Such as my '72 or 73 olds... They put out a constant voltage regardless. Those style systems would work much nicer with a multi battery system because they would ALWAYS charge the batteries fully. Manufacturers only went to the voltage sensing alternators to save gas since the alternator wouldn't be running full bore all the time. Some race cars have clutches on their alternators that disengage the alternators for high speed drag runs too.

Oh please Red - how can two batteries connected in parallel have different voltages?
DO YOU NOT UNDERSTAND BASIC ELECTRICAL CIRCUIT THEORY?

I'll put alternators into simple terms. They maintain a regulated voltage. They (try to) maintain up to 14.4V, typically 14.2V. In ye olde days that figure was 13.8V.
The only other difference is that some alternators extend their voltage sensing to (f.ex) the battery whilst others (like D+ and older alternator/regulators) sense the alternator output voltage.
But they are all "voltage sensing" in that if that "sensed" voltage drops, they increase rotor current to compensate, and vice versa.

And you are not biting. I wouldn't even call it a bark.

Now please, for once - stop and listen - and THINK!.
Once you realise that paralleled circuits have THE SAME voltage all I've said should finally make sense.

In case you still don't get it....
IS THERE ANYONE HERE THAT AGREES with Red that paralleled batteries will have different voltages?
Is there anyone that agrees with me?
Is it time we ran a poll?

if you connect 2 batteries with different charge levels in parallel, higher voltage battery will start charging the lower voltage battery. the current from the higher voltage battery to the lower voltage battery will continue to flow until the voltages equalize. of course, when they are connected in parallel, the potential difference is same between the poles of individual batteries but as soon as they are disconnected from each other (before the current flow goes to zero), the voltages of the batteries will not be same anymore.
looking from the alternator point of view, it will monitor the voltage of its output, which means actually the voltage of the battery that its charging. same story, if connected parallel (alternator and the battery(ies)), then the voltages will be the same, excluding the difference caused by current and cable thickness and length, in other words voltage drop.
a fully charged battery (14.4V) can NEVER maintain the same voltage when its connected to a less charged battery in parallel. which in this case alternator will try to increase the voltage at its output to 14.4V. meaning, it will charge both of the batteries until the voltage of parallel connected battery group's voltage rises up to 14.4V(only possible when both of the batteries are fully charged).
ps. fully charged battery is considered as 14.4V in my text, but different applications consider different battery voltage levels as fully charged.

coming to the main question,
I would try to solve the main problem instead of trying to cover the symptoms with different remedies.

Thank you all for your inputs! I do plan on fixing the headlight issue, but thats another can of worms for another forum. I have been planning on doing a second battery installation for quite a while now. In fact, I had all of the parts I needed except the battery this morning. I picked up a battery this afternoon and reran all of my cables. So far so good, I just need to find something to tap into for my remote. It needs to read 12V+ when the car is on, and 0 when the car is in accessory & off. Any ideas?

The headlight is probably a bad connection or motor problem. (Rotate fuses; disconnect & reconnect all connectors; maybe spray the motor/gear/hinge area.)
the12volt.com may be good for that issue.

And with apologies for maintaining this hijack, but a correction....

canstb - a fully charged 12V lead-acid battery is ~12.7V, not 14.xV. 14V is voltage that the alternator or charger raises the battery terminals to.
What you say about paralleling of batteries is totally correct. And because both are getting above 13V, it is impossible for one battery to discharge into another (surface charge excluded).
Thanks for your replay (and support).